Answers

The Brainliest Answer!
2015-06-10T19:12:04+08:00

This Is a Certified Answer

×
Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
Formula:
a_1)}{2} [/tex]
n= \frac{a_n-a_1}{d} +1

1. There is obviously 50 terms here.
So: 
S_n= \frac{n(a_n+[tex]S_{50}= \frac{50(50+1)}{2} =25*51=1275

2. The first term would be 4, and the last would be 80. The common difference is 4.
n= \frac{80-4}{4} +1= 20
S_{20}= \frac{20(80+4)}{2} =840
2 5 2